It is known that spinal musculature does little to stabilize facet joint articulation in the spine. This function is provided by the ligament structure. Most artificial disk replacements today are for degenerate disks which are causing pain. The bone, muscle and the ligaments are all intact, and the spine itself is stable. When a spine is unstable and the bones and ligaments allow the spine to move out of alignment, rods and screws are used to provide stability. In some situations a spinal fusion is performed.
Spinal prostheses for replacement of missing or excised disk material that replicate the functions of the missing tissue have been sought. The CHARITÉ® Artificial Disc was developed at the CHARITÉ University Hospital in Berlin, Germany in the mid-1980s. Further refinements to the design were incorporated with the cooperation of the staff at Waldemar Link GmbH. The basic device includes a sliding core made from a medical grade plastic, sandwiched between two metal endplates made from medical grade cobalt chromium alloy. The endplates support the core and have small teeth which secure them to the vertebrae above and below the disk space. In a clinical study, patients were observed to have motion between 0° and 21° while bending forward and backward. U.S. Pat. No. 4,759,766 for “Intervertebral Disc Endoprosthesis” which issued to Karin Büttner-Janz et al. on Jul. 26, 1988; U.S. Pat. No. 5,401,269 for “Intervertebral Disc Endoprosthesis” which issued to Karin Büttner-Janz et al. on Mar. 28, 1995; and U.S. Pat. No. 5,556,431 for “Intervertebral Disc Endoprosthesis” which issued to Karin Büttner-Janz on Sep. 17, 1996 represent several embodiments of this disk.
U.S. Pat. No. 5,683,465 for “Artificial Intervertebral Disk Prosthesis” which issued to Gary Lee Shinn et al. on Nov. 4, 1997 includes a first disk half having a portion of a socket attached to a first plate, a second disk half having a portion of a ball attached to a second plate, the socket further having a plurality of expansion slots which expand to allow the portion of a ball to be inserted into the socket such that the expansion slots expand around the portion of a ball to be inserted into the socket and then contract to retain the portion of a ball therein. The disk is fastened to the vertebrae by pins, tabs or by a first annulus or second annulus, each of which extend from the disk. A flexible substantially toroidal enclosure is attached to the perimeter of the first plate and to the perimeter of the second plate when desired to form a barrier between the disk and any proximal organic material.
U.S. Pat. No. 7,195,644 for “Ball And Dual Socket Joint” which issued to Robert Diaz et al. on Mar. 27, 2007, U.S. Patent Application Publication 2006/0190084 (Aug. 24, 2006) for “Interior Insert Ball And Dual Socket Joint” by Robert Doubler et al., and U.S. Patent Publication No. 2007/0162133 (Jul. 12, 2007) by Robert Doubler et al. teach a spinal implant inserted between adjacent vertebrae to function as a disk prosthesis which includes two plates fastened to adjacent vertebrae facing each other, the facing sides of the plates each having a depending skirt formed as concentric arcs and bowed or tapered in the axial direction. Depressions are centrally located between the arcs of the plates and a ceramic ball is universally movable in the depressions.
U.S. Pat. No. 7,282,065 for “Disk Augmentation System And Method” which issued to David Louis Kirschman on Oct. 16, 2007 describes a retainer including artificial supports posterior to or exterior of a spinal column and a spinal column axis for receiving and securing at least one artificial compression body in a supported relationship to facilitate reducing the load on one or more natural disks of a patient.
With the exceptions of the '065 and the '644 patents, the above patents and patent applications provide no stability to the spine in the situation where stability is not provided by existing ligaments. The '065 patent does not permit significant movement of the vertebrae involved. The '644 patent teaches that the facing sides of the plates each have a depending skirt formed as concentric depending and upstanding arcs of about 90 degrees, such that each interrupted skirt may be oriented whereby depending opposed arcs are aligned with upstanding opposed arcs, thereby interlocking the plates in a movable joint that cannot be separated axially. The contacting surfaces are spherical or bowed from the plate at least to the height of the diameter of the ball. This design does not permit lateral movement between the plates.